1. Field of the Invention
This invention relates to a synchronization detection device and its method, and more particularly, is suitably applied to a cellular radio communication system that allows asynchronous communication between base stations according to the direct sequence-code division multiple access (DS-CDMA) system.
2. Description of the Related Art
The DS-CDMA system is a multiplexing system using spread codes and its application to the cellular radio communication system has been vigorously under study as one of the radio access systems of the future mobile communication system. In the cellular radio communication system, an area to provide communication service is divided into cells with a desired size and a base station as a fixed station is provided in each cell and a communication terminal device which is a mobile station is connected by radio to the base station having the best communication state.
In such a cellular radio communication system, a method to search a base station to which the mobile station is connected is generally called a cell search. In this DS-CDMA cellular radio communication system, in order that the base stations use the same frequency, timing of spread code included in the received signal should be trapped simultaneously with this cell searching.
This cellular radio communication system using the DS-CDMA system can be classified into two types: a synchronous system between base stations in which temporal synchronization is performed among all base stations; and asynchronous system between base stations in which time synchronization is not conducted. Since the synchronous system between base stations is regulated by the IS-95 Standard, an absolute reference time is set in each base station using radio waves of the global positioning system (GPS) and thus, temporal synchronization will be performed among base stations. In this system, base stations transmit the same long code as the spread code at timing different from each other based on the absolute reference time. And thus, at the time of cell search, the mobile station can search the base station to be connected only by trapping the timing of a long code.
On the other hand, in the asynchronous system between base stations, base stations transmit a different long code in order to identify base stations, and accordingly, at the time of cell search, it is necessary for the mobile station to detect timing of the long code as well as the type of the long code. Therefore, in the case of asynchronous system between base stations, there is a problem that the time required for cell search becomes longer as compared with the synchronous system between base stations. However, as contrary to the above, in the asynchronous system between base station, since it is unnecessary to receive GPS radio wave, service area can be widened to areas where GPS radio wave can not reach. Therefore, if the cell search problem can be solved, this system is very effective.
As a method to speed up the cell search in the asynchronous system between base stations, several methods can be considered. One of these methods is that of transmitting a common short code among base station as well as a long code and a group identification short code to specify the long code group, and of detecting the timing and code type of the long code to be transmitted based on these short codes. In the following explanations, the method to detect the code timing and code type will be referred to as identification.
More specifically, the base station has a signal generation unit 1 of the control channel as shown in FIG. 1, and it forms transmission data in which long code, common short code and group identification short code are combined by using the signal generation unit 1, and transmits the transmission data through the control channel. At first, a first multiplier 2 sequentially spreads input information bit S1 having the value such as xe2x80x9c1xe2x80x9d with the common short code CSC having comparatively short cycle, that is common in each base station, and outputs spread data S2 to a second multiplier 3. In the second multiplier 3, long code LC having the longer cycle than the common short code CSC is entered and the spread data S2 is successively spread using the long code LC and the spread data S3 is output to an adder 5.
In this connection, this long code LC is very specific to every base station and base stations are identified by this long code LC. And a long code enable signal SCE is entered into AND circuit 4, and by setting this long code enable signal LCE to the level xe2x80x9cLxe2x80x9d at a fixed cycle, the long code LC to be supplied to the second multiplier 3 will be masked over the segment of level xe2x80x9cLxe2x80x9d. Thus, the spread data S3 to be sent from the second multiplier 3 is not spread out in the long code LC over the segment on which long code enable signal LCE has the level xe2x80x9cLxe2x80x9d. Hereinafter, the segment over which the long code LC is masked is referred to as masked segment.
On the other hand, group identification short code GISC, that shows the group of long codes LC to be used in the signal generation unit 1 and has the same cycle as the common short code CSC, is entered into the third multiplier 6. And this third multiplier 6 spreads out information bit S4 having such as the value xe2x80x9c1xe2x80x9d with this group identification short code GISC and outputs spread data S5 to the adder 5. In this connection, the spread data S5 will be formed on the masked segment of long code LC.
The adder 5, by adding these spread data S3 and S5, forms transmission data S6 for transmitting by the control channel. Thus, by transmitting this transmission data S6 by the control channel via the transmission circuit and antenna (not shown in FIG.), the transmission signal containing long code LC, common short code CSC and group identification short code GISC is be transmitted from the base station.
At this point, the timing of long code LC, common short code CSC and group identification short code GISC included in the transmission signal transmitted from the base station will be shown in FIGS. 2A to 2C. As shown in FIGS. 2A to 2C, common short codes CBC exist repeatedly in the transmission signal. Also long codes LC exist repeatedly in the transmission signal. However, the long code LC is masked just over the segment synchronized with the common short code CSC at the cycle TMK. Moreover, in the masked segment of the long code LC, since the spread data S5 is added, group identification short code GISC exists over that masked segment.
In the case of receiving the transmission signal containing codes (CSC, GISC and LC) by the mobile station at the above timing and identifying the long code LC included in the transmission signal, firstly the common short code CSC existing over the masked segment is detected from the received signal to detect the timing of long code LC. When it is detected, the type of group identification short code GISC existing over the masked segment is determined. In this case, the group identification short code GISC shows the group of long code LC included in the received signal, and if the type of group identification short code GISC can be identified, the candidate of long code LC can be specified to that group.
Accordingly, after the group identification short code GISC is determined, the type of long code LC included in the received signal can be identified by narrowing down the candidate onto the long code LC in the group which the group identification short code GISC indicates and sequentially confirming whether these are candidates or not. With this arrangement, since the number of candidates can be decreased by the group identification short code GISC, the time required to judge the type of long code LC can be shortened as compared with the case of making all long codes LC as candidates.
At this point, the synchronization detection device to identify long code LC included in the received signal according to the above method will be shown in FIG. 3. In this FIG. 3, 10 generally shows a synchronization detection device to be provided in the mobile station, receiving the received signal S10 received via an antenna and a receiver (not shown in FIG.), and as well as detecting the timing of long code LC having the strongest signal level included in the received signal S10, it determines the type of that long code LC. More specifically, at the time of cell search, this judges the base station by determining the timing and code type of the long code LC having the strongest signal level.
At first, the matched filter 11 detects the correlation values between the received signal S10 and the replica code DCSC of the common short code CSC generated at the short code generator 12 in succession and stores the correlation value data S11 in a memory 13. The matched filter 11 detects the correlation value at least over the period of approximately three cycles of long code cycle.
The maximum correlation detection circuit 14 reads out the correlation value data S11 stored in the memory 13 and detects the data having the largest correlation value among the data S11. And assuming the timing at which the largest correlation value is obtained as the timing of long code LC having the strongest signal level included in the received signal S10, the maximum correlation detection circuit 14 outputs timing information S12 showing that timing. This timing information S12 is transmitted to the short code generator 12 and the long code generator 15 as the timing information S12 to generate replica code of the group identification short code GISC and replica code of the long code LC. Moreover, the maximum correlation detection circuit 14 outputs the detected correlation value data S13 having the largest value to a threshold determining circuit 18.
When the short code generator 12 receives the timing information S12, it generates replica code DGISC, that is the first candidate in the plural number of group identification short codes GISC at the timing shown by the timing information S12 and outputs the code DGISC is to a sliding correlator 17 via a multiplier 16.
On the other hand, the threshold determining circuit 18 determines the first threshold value for determining the type of group identification short code GISC and the second threshold value to determine the type of long code LC based on the correlation value data S13 and outputs these to a judging unit 19 as the threshold data S14.
The sliding correlator 17 successively multiplies the replica code DGISC of the group identification short code GISC by the input received signal S10 and by integrating the multiplication result for 1 cycle of the replica code DGISC, calculates the correlation value and outputs the correlation value data S15 to the judging unit 19.
The judging unit 19 judges whether the correlation data S15 transmitted from the sliding correlator 17 exceeds the first threshold value or not, and if it does not exceed the first threshold value, the control signal S16 is output to the short code generator 12, and causes this short code generator 12 to generate replica code DGISC, that is the next candidate of the group identification short code GISC. Thus, the replica code DGISC of the group identification short code GISC is to be generated by the short code generator 12 in succession and the correlation value data S15 of that replica code DGISC is obtained successively by the sliding correlator 17.
On the contrary, if the correlation value data S15 transmitted from the sliding correlator 17 exceeds the first threshold value, the judging unit 19 judges that the then replica code DGISC is the group identification short code GISC showing the group of long codes LC to be detected and outputs the group shown by that group identification short code GISC to the long code generator 15 as group information S17. Also, in the case where the correlation value data S15 exceeds the first threshold value, the judging unit 19 outputs a control signal S18 to the short code generator 12 and causes the short code generator 12 to generate replica code DCSC of the common short code CSC.
When the long code generator 15 receives the group information S17, it generates replica code DLC that is the first candidate of the long code LC in the group which the group information S17 idicates at the timing shown by timing information S12. This replica code DLC of the long code LC is entered into the sliding correlator 17 after being multiplied by replica code DCSC of the common short code CSC in the multiplier 16.
The sliding correlator 17 successively multiplies the input received signal S10 by the replica code DLC of the long code LC which has been multiplied by the replica code DCSC of the common short code CSC and by integrating the multiplication result for 1 cycle of replica code DLC, calculates the correlation value and outputs the correlation value data S19 to the judging unit 19.
The judging unit 19 judges whether the correlation value data S19 output from the sliding correlator 17 exceeds the second threshold value or not, and if it does not exceed, a control signal S20 is output to the long code generator 15, and causes this to generate the next candidate of the long code LC, i.e., replica code DLC. Thus, the judging unit 19 causes the long code generator 15 to generate replica code DLC of the long code LC in succession and obtains the correlation value data S19 of that replica code DLC by the sliding correlator 17.
On the contrary, in the case where the correlation value data S19 to be sent out from the sliding correlator 17 exceeds the second threshold value, the judging unit 19 judges the then replica code DLC as the long code LC to be detected and outputs information S21 showing the type of the long code LC. Accordingly, this synchronization detection device 10, at the first stage, by detecting the common short code CSC, detects the timing of the long code LC having the strongest signal level in the received signal. At the following second stage, the judging unit 19 identifies the group identification short code GISC included in the received signal to detect the group of long code LC to be detected, and at the third stage, it determines the type of long code LC by making the long code LC in that group as a candidate. Thus, in this synchronization detection device 10, the long code LC having the strongest signal level included in the received signal can be identified.
At this point, the construction of matched filter 11 for detecting the correlation value of the common short code CSC shown in FIG. 3 will be shown in FIG. 4. Since generally the received signal S10 is the quadrature-phase-shift-keying (QPSK)-modulated, in practice the matched filter 11 has the 4-phase construction as shown in this FIG. 4. First, in the matched filter 11, the received signal S10 is entered into multipliers 20 and 21. in the multiplier 20, carrier signal 826, which is generated by delaying the carrier signal S25 generated in the oscillator 22 by xcfx80/2 with the phase-shifter 23, is entered. The multiplier 20, by multiplying this carrier signal S26 by the received signal S10, takes out signal element SI of the in-phase element I in the received signal S10. This signal element SI of the in-phase element I, after its unnecessary element is eliminated through the low-pass filter 24, is entered into an analog-to-digital converter 25 to be converted to digital in-phase data UI.
On the other hand, the carrier signal S25 generated in the oscillator 22 is entered into the multiplier 21. The multiplier 21, by multiplying the carrier signal S25 by the received signal S10, takes out a signal element SQ of the quadrature element Q included in the received signal S10. This signal element SQ of the quadrature element Q, after its unnecessary element is eliminated through the low-pass filter 24, will be entered into an analog-to-digital converter 27 and digital conversion is applied here And converted to digital quadrature data UQ.
Correlators 28 to 31 are matched filters for detecting the correlation value per each signal element. Of replica code DCSC of the common short code CSC to be transmitted from the short code generator 22, the in-phase data UI and the replica code UIR of in-phase element are entered into the correlator 28 and the correlator 28 calculates the correlation value UII (=UI*UIR) between the in-phase data UI and the in-phase element replica code UIR, and outputs this to an adder 32.
Furthermore, in the correlator 29, the in-phase data UI and the orthogonal element replica code UQR of replica code DCSC of the common short code CSC to be transmitted from the short code generator 12 are entered. The correlator 29 calculates the correlation value UIQ (=UI*UQR) between the in-phase data UI and the quadrature element replica code UQR and outputs this to a differentiator 33.
Similarly, in the correlator 30, the above-mentioned quadrature data UQ and the replica code UQR of the quadrature element are entered. The correlator 30 calculates the correlation value UQQ (=UQ*UQR) between the quadrature data UQ and quadrature element replica code UQR, and outputs this to the adder 32. Moreover the above-mentioned quadrature data UQ and the in-phase element replica code UIR are entered in the correlator 31 to calculate the correlation value UQI (=UQ*UIR) between the quadrature data UQ and the in-phase element replica code UIR and output this to the differentiator 33.
The adder 32 adds up the correlation value UII and the correlation value UQQ and outputs the sum to a square-law circuit 34. On the other hand, the differentiator 33 calculates the difference between the correlation value UQI and the correlation value UIQ and outputs the difference to a square-law circuit 35. Thus, the sum and the difference are squared by square-law circuits 34 and 35 respectively and by adding the squared result by an adder 36, finally the correlation value data S11 (=(UII+UQQ)2+(UQIxe2x88x92UIQ)2) to the replica code DCSC, is calculated.
According to the identification method of long code LC as described above, the timing detection processing of long code LC using the common short code CSC, the group identification processing of long code LC using the group identification short code GISC and the identification processing of long code LC focussing the candidate onto the identified group are conducted in time series using the matched filter and the sliding correlator, and basically each processing is conducted at different timing. When the condition of transmission path changes due to, for example, fading, there is the possibility that the long code LC cannot be identified. Thus, according to the identification method described above, this possibility is avoided by extending the identification period of the long code LC such as detecting the correlation values of the overall long codes LC, but it causes the inconvenience of taking time when identifying long codes LC.
In view of the foregoing, an object of this invention is to provide a synchronization detection device which can accurately identify codes to be detected at high speed and can minimize the circuits.
The foregoing object and other objects of the invention have been achieved by the provision of a synchronization detection device for receiving a signal containing the first code, the known second code to detect the timing of the first code and the third code to specify the group of the first code and for detecting the timing and code type of the first code included in the received signal. The synchronization detection device comprises a matched filtering means for receiving the first, the second or the third replica code corresponding to the first, the second or the third code and detecting the correlation value between the first, the second or the third replica code and the received signal as well as capturing the received signal based on the data shift clock to be supplied; a correlation coefficient generation means for generating the first, the second or the third replica code and supplying it to the matched filtering means; and a control means for stopping the supply of data shift clock at the desired timing and causing the matched filtering means to hold the received signal, switching the replica code to be generated at the correlation coefficient generation means to the first, the second or the third replica code at desired timing and detecting the then correlation value, and thereby detecting the second code, the third-code and the first code successively and detecting the timing and code type of the first code.
Thus, the data shift clock to be given to the matched filtering means is stopped at desired timing and the received signal is held, and the replica code generated at the correlation coefficient generation means is switched to the first, the second or the third replica code at desired timing to detect the then correlation value. Thereby, the second code, the third code and the first code are successively detected and the timing and code type of the first code are detected. Therefore, the matched filtering means can conduct the correlation detection at high speed holding the received signal and each correlation detection can be conducted at approximately the same timing, and thereby the first code included in the received signal can be identified at high speed as compared with the conventional device.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.